Electrode with redundant impedance reduction

ABSTRACT

An electrode assembly that includes an electrically conductive layer, a first impedance reduction system, and a second impedance reduction system. The electrically conductive layer forms an electrode portion of the electrode assembly and a first surface to be placed adjacent a person&#39;s skin. The first impedance reduction system is configured to dispense a first amount of an electrically conductive gel onto the first surface of the electrically conductive layer in response to a first activation signal. The second impedance reduction system is configured to dispense a second amount of the electrically conductive gel onto the first surface of the electrically conductive layer in response to a second activation signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §120 as acontinuation of U.S. application Ser. No. 13/849,751 titled “ELECTRODEWITH REDUNDANT IMPEDANCE REDUCTION” filed Mar. 25, 2013, which claimsthe benefit under 35 U.S.C. §120 as a continuation of U.S. applicationSer. No. 13/315,937 titled “ELECTRODE WITH REDUNDANT IMPEDANCEREDUCTION” filed Dec. 9, 2011, which claims priority under 35 U.S.C.§120 as a continuation of International Patent Application Serial No.PCT/US2011/063931, titled “ELECTRODE WITH REDUNDANT IMPEDANCE REDUCTION”filed Dec. 8, 2011, which claims priority under 35 U.S.C. §119(e) toU.S. Provisional Application Ser. No. 61/421,283 titled “ELECTRODE WITHREDUNDANT IMPEDANCE REDUCTION” filed Dec. 9, 2010. U.S. application Ser.No. 13/315,937 titled “ELECTRODE WITH REDUNDANT IMPEDANCE REDUCTION”filed Dec. 9, 2011, also claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 61/421,283 titled “ELECTRODE WITHREDUNDANT IMPEDANCE REDUCTION” filed Dec. 9, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to medical electrodes, and moreparticularly, to a medical electrode with redundant impedance reductionsystems that may be used with a wearable medical device, such as adefibrillator.

2. Discussion of Related Art

Cardiac arrest and other cardiac health ailments are a major cause ofdeath worldwide. Various resuscitation efforts aim to maintain thebody's circulatory and respiratory systems during cardiac arrest in anattempt to save the life of the victim. The sooner these resuscitationefforts begin, the better the victim's chances of survival. Theseefforts are expensive and have a limited success rate, and cardiacarrest, among other conditions, continues to claim the lives of victims.

To protect against cardiac arrest and other cardiac health ailments,some at-risk patients may use a wearable defibrillator, such as theLifeVest® wearable cardioverter defibrillator available from ZollMedical Corporation of Chelmsford, Mass. To remain protected, thepatient wears the device nearly continuously while going about theirnormal daily activities, while awake, and while asleep.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an electrodeassembly is provided. The electrode assembly comprises an electricallyconductive layer that forms an electrode portion of the electrodeassembly, a first impedance reduction system, and a second impedancereduction system. The electrically conductive layer has a first surfaceto be placed adjacent a person's skin. The first impedance reductionsystem is configured to dispense a first amount of a first electricallyconductive gel onto the first surface of the electrically conductivelayer in response to a first activation signal, and the second impedancereduction system is configured to dispense a second amount of a secondelectrically conductive gel onto the first surface of the electricallyconductive layer in response to a second activation signal.

In accordance with one embodiment, the first activation signal and thesecond activation signal are based upon the same signal. In accordancewith another embodiment, the first activation signal is distinct fromthe second activation signal.

In accordance with an aspect of the present invention, the secondimpedance reduction system is configured to dispense the second amountof the second electrically conductive gel onto the first surface of theelectrically conductive layer in response to the second activationsignal and independent of whether the first impedance reduction systemdispenses the first amount of the first electrically conductive gel onthe first surface of the electrically conductive layer in response tothe first activation signal.

In accordance with one embodiment, the first impedance reduction systemis similar in construction to the second impedance reduction system.

In accordance with one embodiment, the electrically conductive layer hasa plurality of apertures formed there through, the plurality ofapertures including a first plurality of apertures and a secondplurality of apertures. In accordance with this embodiment, the firstimpedance reduction system is configured to dispense the first amount ofthe first electrically conductive gel through the first plurality ofapertures and onto the first surface of the electrically conductivelayer in response to the first activation signal, and the secondimpedance reduction system is configured to dispense the second amountof the second electrically conductive gel through the second pluralityof apertures and onto the first surface of the electrically conductivelayer in response to the second activation signal. In one embodiment,the first electrically conducting gel and the second electricallyconducting gel are the same type of electrically conducting gel.

In accordance with another aspect of the present invention, an electrodeassembly is provided. The electrode assembly comprises an electricallyconductive layer, a first plurality of gel reservoirs, a secondplurality of gel reservoirs, a first fluid channel, a second fluidchannel, a first fluid pressure source, and a second fluid pressuresource. The electrically conductive layer has a plurality of aperturesformed therein, the plurality of apertures including a first pluralityof apertures and a second plurality of apertures. The first plurality ofgel reservoirs each contain a first electrically conductive gel, eachrespective gel reservoir of the first plurality of gel reservoirs havingan outlet to fluidly communicate with a respective aperture of the firstplurality of apertures. The second plurality of gel reservoirs eachcontain a second electrically conductive gel, each respective gelreservoir of the second plurality of gel reservoirs having an outlet tofluidly communicate with a respective aperture of the second pluralityof apertures. The first fluid channel is in fluid communication witheach of the first plurality of gel reservoirs, and the second fluidchannel is in fluid communication with each of the second plurality ofgel reservoirs. The first fluid pressure source is in fluidcommunication with the first fluid channel and configured to receive afirst activation signal and force a first fluid under pressure into thefirst fluid channel in response to the first activation signal, and thesecond fluid pressure source is in fluid communication with the secondfluid channel and configured to receive a second activation signal andforce a second fluid under pressure into the second fluid channel inresponse to the second activation signal.

In accordance with one embodiment, the outlet of each of the firstplurality of gel reservoirs and the outlet of each of the secondplurality of gel reservoirs is sealed by a membrane that is constructedto rupture in response to the pressure of the first fluid and the secondfluid, respectively.

In accordance with one embodiment, the first activation signal and thesecond activation signal are based upon the same signal. In anotherembodiment, the first activation signal is distinct from the secondactivation signal.

In accordance with another aspect of the present invention, an electrodeassembly is provided that comprises at least one ECG sensing electrode,a therapy electrode, a first impedance reduction system and a secondimpedance reduction system. The at least one ECG sensing electrode isconfigured to monitor an ECG signal of a patient and the therapyelectrode is configured to deliver a defibrillating shock to thepatient. The first impedance reduction system is configured to reduce animpedance between the therapy electrode and the patient in response to afirst activation signal, and the second impedance reduction system isconfigured to reduce the impedance between the therapy electrode and thepatient in response to a second activation signal. In accordance withone embodiment, the at least one ECG sensing electrode is electricallyinsulated from the therapy electrode.

In accordance with one embodiment, the at least one ECG sensingelectrode includes a plurality of ECG sensing electrodes. In accordancewith a further aspect of this embodiment, each of the plurality of ECGsensing electrodes is electrically insulated from the therapy electrode.

In accordance with one embodiment, the electrode assembly furthercomprises at least one additional sensor configured to monitor aphysiological parameter of the patient that is other than the ECG signalof the patient.

In accordance with one embodiment, the first activation signal and thesecond activation signal are based upon the same signal, and the firstimpedance reduction system is similar in construction to the secondimpedance reduction system.

In accordance with one embodiment, the therapy electrode includes anelectrically conductive layer having a first surface to be placedadjacent a person's skin and a plurality of apertures formed through theelectrically conductive layer. The plurality of apertures includes afirst plurality of apertures and a second plurality of apertures. Thefirst impedance reduction system is configured to dispense a firstamount of a first electrically conductive gel through the firstplurality of apertures and onto the first surface of the electricallyconductive layer in response to the first activation signal, and thesecond impedance reduction system is configured to dispense a secondamount of a second electrically conductive gel through the secondplurality of apertures and onto the first surface of the electricallyconductive layer in response to the second activation signal.

In accordance with another aspect of the present invention, a method ofreducing impedance between an electrode and a patient's skin isprovided. The method comprises acts of sending a first activation signalto a first impedance reduction system configured to dispense a firstamount of a first electrically conducting gel onto a surface of theelectrode that is configured to be disposed adjacent the patient's skin,and sending a second activation signal to a second impedance reductionsystem configured to dispense a second amount of a second electricallyconducting gel onto the surface of the electrode that is configured tobe disposed adjacent the patient's skin, the second impedance reductionsystem being distinct from the first impedance reduction system.

In accordance with one embodiment, the method further comprises an actof determining whether the first impedance reduction system dispensedthe first amount of the first electrically conducting gel onto thesurface of the electrode, wherein the act of sending the secondactivation signal to the second impedance reduction system is performedin response to a determination that the first impedance reduction systemdid not dispense the first amount of the first electrically conductinggel onto the surface of the electrode. In accordance with oneembodiment, the act of sending the second activation signal is performedsubsequent to the act of sending the first activation signal. In analternative embodiment, the act of sending the second activation signalis performed substantially simultaneously with the act sending the firstactivation signal.

Still other aspects, embodiment, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Moreover, it isto be understood that both the foregoing information and the followingdetailed description are merely illustrative examples of various aspectsand embodiments of the present invention, and are intended to provide anoverview or framework for understanding the nature and character of theclaimed aspects and embodiments. Any embodiment disclosed herein may becombined with any other embodiment in any manner consistent with aspectsof the present invention disclosed herein, and references to “anembodiment,” “some embodiments,” “an alternate embodiment,” “variousembodiments,” “one embodiment,” “at least one embodiment,” “this andother embodiments” or the like are not necessarily mutually exclusiveand are intended to indicate that a particular feature, structure, orcharacteristic described in connection with the embodiment may beincluded in at least one embodiment. The appearance of such terms hereinis not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 illustrates a wearable medical device, such as a wearabledefibrillator;

FIG. 2 a is a plan view of an electrode portion of a therapy electrodeassembly that may be used with the wearable medical device illustratedin FIG. 1;

FIG. 2 b is a functional block diagram of an impedance reduction systemthat may be included in the electrode portion of FIG. 2 a;

FIG. 3 is a functional block diagram of a redundant impedance reductionsystem in accordance with an aspect of the present invention;

FIG. 4 is a schematic diagram of an electrode assembly that includes ECGsensing electrodes, a therapy electrode, and redundant impedancereduction systems in accordance with another aspect of the presentinvention; and

FIG. 5 illustrates the manner in which the electrode assembly of FIG. 4may be worn on the body of a patient.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

FIG. 1 illustrates a wearable medical device, such as a LifeVest®wearable cardioverter defibrillator available from Zoll MedicalCorporation of Chelmsford, Mass. As shown, the wearable medical device100 includes a harness 110 having a pair of shoulder straps and a beltthat is worn about the torso of a patient. The harness 110 is typicallymade from a material, such as cotton, that is breathable, and unlikelyto cause skin irritation, even when worn for prolonged periods of time.The wearable medical device 100 includes a plurality of ECG sensingelectrodes 112 that are attached to the harness 110 at various positionsabout the patient's body and electrically coupled to a control unit 120via a connection pod 130. The plurality of ECG sensing electrodes 112,which may be dry-sensing capacitance electrodes, are used by the controlunit 120 to monitor the cardiac function of the patient and generallyinclude a front/back pair of ECG sensing electrodes and a side/side pairof ECG sensing electrodes. Additional ECG sensing electrodes may beprovided, and the plurality of ECG sensing electrodes 112 may bedisposed at varying locations about the patient's body.

The wearable medical device 100 also includes a plurality of therapyelectrodes 114 that are electrically coupled to the control unit 120 viathe connection pod 130 and which are capable of delivering one or moretherapeutic defibrillating shocks to the body of the patient, if it isdetermined that such treatment is warranted. As shown, the plurality oftherapy electrodes 114 includes a first therapy electrode 114 a that isdisposed on the front of the patient's torso and a second therapyelectrode 114 b that is disposed on the back of the patient's torso. Thesecond therapy electrode 114 b includes a pair of therapy electrodesthat are electrically coupled together and act as the second therapyelectrode 114 b. The use of two therapy electrodes 114 a, 114 b permitsa biphasic shock to be delivered to the body of the patient, such that afirst of the two therapy electrodes can deliver a first phase of thebiphasic shock with the other therapy electrode acting as a return, andthe other therapy electrode can deliver the second phase of the biphasicshock with the first therapy electrode acting as the return. Theconnection pod 130 electrically couples the plurality of ECG sensingelectrodes 112 and the plurality of therapy electrodes 114 to thecontrol unit 120, and may include electronic circuitry. For example, inone implementation the connection pod 130 includes signal acquisitioncircuitry, such as a plurality of differential amplifiers to receive ECGsignals from different ones of the plurality of ECG sensing electrodes112 and to provide a differential ECG signal to the control unit 120based on the difference therebetween. The connection pod 130 may alsoinclude other electronic circuitry, such as a motion sensor oraccelerometer by which patient activity may be monitored.

As shown in FIG. 1, the wearable medical device 100 also includes a userinterface pod 140 that is electrically coupled to the control unit 120.The user interface pod 140 can be attached to the patient's clothing orto the harness 110, for example, via a clip (not shown) that is attachedto a portion of the interface pod 140. Alternatively, the user interfacepod 140 may simply be held in a person's hand. In some embodiments, theuser interface pod 140 may communicate wirelessly with the control unit120, for example, using a Bluetooth®, Wireless USB, ZigBee, WirelessEthernet, GSM, or other type of communication interface. The userinterface pod 140 typically includes a number a number of buttons bywhich the patient, or a bystander can communicate with the control unit120, and a speaker by which the control unit 120 may communicate withthe patient or the bystander. For example, where the control unit 120determines that the patient is experiencing cardiac arrhythmia, thecontrol unit 120 may issue an audible alarm via a loudspeaker (notshown) on the control unit 120 and/or the user interface pod 140alerting the patient and any bystanders to the patient's medicalcondition. The control unit 120 may also instruct the patient to pressand hold one or more buttons on the control unit 120 or on the userinterface pod 140 to indicate that the patient is conscious, therebyinstructing the control unit 120 to withhold the delivery of one or moretherapeutic defibrillating shocks. If the patient does not respond, thedevice may presume that the patient is unconscious, and proceed with thetreatment sequence, culminating in the delivery of one or moredefibrillating shocks to the body of the patient. In some embodiments,the functionality of the user interface pod 140 may be integrated intothe control unit 120.

The control unit 120 generally includes at least one processor,microprocessor, or controller, such as a processor commerciallyavailable from companies such as Texas Instruments, Intel, AMD, Sun,IBM, Motorola, Freescale and ARM Holdings. In one implementation, the atleast one processor includes a power conserving processor arrangementthat comprises a general purpose processor, such as an Intel® PXA270processor and a special purpose processor, such as a Freescale™ DSP56311Digital Signal Processor. Such a power conserving processor arrangementis described in co-pending application Ser. No. 12/833,096, entitledSYSTEM AND METHOD FOR CONSERVING POWER IN A MEDICAL DEVICE, filed Jul.9, 2010 (hereinafter the “'096 application”) which is incorporated byreference herein in its entirety. The at least one processor of thecontrol unit 120 is configured to monitor the patient's medicalcondition, to perform medical data logging and storage, and to providemedical treatment to the patient in response to a detected medicalcondition, such as cardiac arrhythmia. Although not shown, the wearablemedical device 100 may include additional sensors, other than the ECGsensing electrodes 112, capable of monitoring the physiologicalcondition or activity of the patient. For example, sensors capable ofmeasuring blood pressure, heart rate, thoracic impedance, pulse oxygenlevel, respiration rate, heart sounds, and the activity level of thepatient may also be provided.

As discussed above, to provide protection against cardiac arrest,patients that use a wearable medical device, such as a wearabledefibrillator, generally wear the device nearly continuously while theyare awake and while they are asleep. Because the wearable medical deviceis worn nearly continuously, dry electrodes are typically used for boththe plurality of ECG sensing electrodes 112 and the plurality of therapyelectrodes 114 for comfort and to prevent irritation of the patient'sskin. Where it is determined that one or more defibrillating shocks areto be delivered to the body of the patient and the patient isnon-responsive, the control unit 120 sends a signal to the plurality oftherapy electrodes 114 causing them to release an impedance reducing gelprior to delivery of one or more defibrillating shocks. The impedancereducing gel reduces the impedance between the conductive surface of thetherapy electrodes and the patient's skin, thereby improving theefficiency of the energy delivered to the patient and reducing thechance of damage (e.g., in the form of burning, reddening, or othertypes of irritation) to the patient's skin.

FIG. 2 a is a plan view of an electrode portion of a therapy electrodeassembly that includes an impedance reduction system and which may beused with a wearable medical device, such as the wearable defibrillatordescribed above with respect to FIG. 1. FIG. 2 b is a functional blockdiagram of the impedance reduction system that is included in theelectrode portion of the therapy electrode assembly shown in FIG. 2 a.The impedance reduction system, when activated, dispenses an impedancereducing (i.e., electrically conductive) gel onto the exposed surface ofthe electrode portion of the therapy electrode assembly that, in use, isplaced most proximate to the patient's body. The electrode portion 200is a multiple layer laminated structure that includes an electricallyconductive layer (not visible in FIG. 2 a, but disposed adjacent thebottom surface of the electrode portion 200 shown in FIG. 2 a) thatforms the electrode and an impedance reduction system 201. In use, theelectrically conductive layer is disposed adjacent the patient's skin,although the conductive layer need not make direct contact with thepatient, as portions of the harness 110 (FIG. 1) and/or portions of thepatient's clothing may be present between the electrically conductivelayer and the patient's skin. As shown in FIG. 2 a, the impedancereduction system 201 is disposed on a side of the electrode portion 200(i.e., the top-side shown in FIG. 2 a) that is opposite the side onwhich the conductive layer is formed.

The impedance reduction system 201 includes a plurality of conductivegel reservoirs 210, each of which has a respective gel delivery outlet220, that are fluidly coupled to a fluid channel 230, and a fluidpressure source 240. The fluid pressure source 240 is fluidly coupled tothe fluid channel 230, and when activated by an activation signal,forces a fluid, such as Nitrogen gas, into the channel 230. Thehydraulic pressure of the fluid from the activated fluid pressure source240 in the fluid channel 230 forces the conductive gel stored in each ofthe plurality of gel reservoirs out of the plurality of gel deliveryoutlets 220 through apertures formed in the electrically conductivelayer and onto the exposed surface of the electrically conductive layerthat, in use, is placed most proximate to the patient's body. Theapertures in the electrically conductive layer are generally alignedwith the plurality of gel delivery outlets 220 so that when activated,the electrically conductive gel is dispensed onto the exposed surface ofthe electrode portion that is disposed most proximate to the patient'sbody. Further details regarding the construction of the electrodeportion 200 are described in U.S. Pat. No. 5,078,134 (hereinafter “the'134 patent”) which is incorporated herein by reference.

Applicants have appreciated that there may be instances where it wouldbe desirable to have redundancy in the impedance reduction systemdescribed above. An electrode that incorporates redundant impedancereduction systems is now described with respect to FIGS. 3-5 below.

FIG. 3 is a functional block diagram of a redundant impedance reductionsystem that may be incorporated in an electrode assembly in accordancewith an aspect of the present invention. As shown, the redundantimpedance reduction system 300 includes at least two independentimpedance reduction systems 301, 302 similar in construction andoperation to that described previously with respect to FIGS. 2 a and 2b. Although only two impedance reduction systems 301, 302 are shown inFIG. 3, it should be appreciated that additional impedance reductionsystems may be provided.

As shown, a first impedance reduction system 301 of the at least twoimpedance reduction systems 301, 302 includes a first plurality of gelreservoirs 310 a, each containing an electrically conductive gel, witheach respective gel reservoir including a gel delivery outlet 320 a.Each of the first plurality of gel reservoirs 310 a is fluidly coupledto a first fluid channel 330 a that is, in turn, fluidly coupled to afirst fluid pressure source 340 a. The first fluid pressure source 340 ahas an input 341 a to receive a first electrical activation signal and afluid outlet 342 a that is fluidly coupled to the first fluid channel330 a. A rupturable membrane and/or a filter (not shown) may bepositioned between the fluid outlet 342 a and the first fluid channel330 a as described in the '134 patent. As described in the '134 patent,the first fluid pressure source 340 a may include a gas generatingcartridge that ignites a chemical pellet (such as a Lead Styphnateigniter and a gas generating mixture of Ammonium Dichromate andNitroguanidine) that rapidly decomposes and generates quantities of agas, such as Nitrogen. It should be appreciated that other types offluid pressure sources may be used, as the present invention is notlimited to any particular type of fluid pressure source.

In response to the first activation signal received at the input 341 aof the first fluid pressure source 340 a, a fluid, such as Nitrogen gas,is forced into the first fluid channel 330 a and then into each of thefirst plurality of gel reservoirs 310 a. The hydraulic pressure of thefluid flowing into each of the first plurality of gel reservoirs 310 aforces the electrically conductive gel contained in each gel reservoirtoward its respective gel delivery outlet 320 a, thereby fracturing amembrane separating the gel delivery outlet from a respective apertureformed in the electrically conductive layer of the electrode portion.

The second impedance reduction system 302 of the at least two impedancereduction systems 301, 302 is similar to the first impedance reductionsystem 301 and includes a second plurality of gel reservoirs 310 b, eachcontaining an electrically conductive gel, with each respective gelreservoir including a gel delivery outlet 320 b. The electricallyconductive gel contained in the second plurality of gel reservoirs 310 bmay, but need not, be the same type of gel as that contained in thefirst plurality of gel reservoirs 310 a. For example, the electricallyconductive gel contained in the second plurality of gel reservoirs 310 bmay have a different color, or have a longer drying time than the gelcontained in the first plurality of gel reservoirs 310 a. Each of theplurality of gel reservoirs 310 b is fluidly coupled to a second fluidchannel 330 b that is, in turn, fluidly coupled to a second fluidpressure source 340 b. The second fluid pressure source 340 b has aninput 341 b to receive a second electrical activation signal and a fluidoutlet 342 b that is fluidly coupled to the second fluid channel 330 b.The second fluid pressure source 340 b may similar in construction tothe first fluid source 340 a described above.

As shown in FIG. 3, the input 341 a of the first fluid pressure source340 a may be electrically connected to the input 341 b of the secondfluid pressure source, such that a single activation signal activateseach of the at least two impedance reduction systems 301, 302substantially simultaneously. Should one of the redundant impedancereduction systems 301, 302 fail to operate (either partially orcompletely), the other can still operate to dispense conductive gel ontothe exposed surface of the electrode. The activation signal provided tothe input 341 a of the first fluid pressure source 340 a may be providedby the control unit 120 (FIG. 1) to the first fluid pressure source 340a using an electrical conductor that is physically distinct from thatwhich provides the activation signal to the input 341 b of the secondfluid pressure source 340 b to permit further redundancy, for example,should one of the electrical conductors be damaged. Alternatively, asingle electrical conductor may be provided between the control unit 120and the electrode assembly, with the single electrical conductor beingconnected to both the input 341 a of the first fluid pressure source 340a and the input 341 b of the second fluid pressure source 340 b.

It should be appreciated that each of the first and second pressuresources 340 a, 340 b may alternatively receive separate activationsignals, as the present invention is not limited to receiving a singleactivation signal. The separate activation signals may be sent, forexample by the control unit 120, to each of the first fluid pressuresource 340 a and the second fluid pressure source 340 b at substantiallythe same time, or at different times. For example, a first activationsignal may be provided to the input 341 a of the first fluid pressuresource 340 a at a first time, and a second activation signal may beprovided to the input 341 b of the second fluid pressure source 340 b ata second time that is subsequent to the first time. In accordance withone embodiment, the control unit 120 (FIG. 1) may send the firstactivation signal to the first fluid pressure source 340 a at a firsttime, and send the second activation signal to the second fluid pressuresource 340 b at a second and subsequent time where it is determined thatthe first impedance reduction system 301 failed to operate.Alternatively, the second activation signal may be sent to the secondfluid pressure source 340 b at a second and subsequent time even whereactivation of the first fluid pressure source 340 a is successful. Sucha subsequent activation of the second fluid pressure source 340 b wouldpermit a second deployment of conductive gel onto the exposed surface ofthe electrode and permit the electrode to maintain a high conductivitywith the patient for a longer period of time than if both impedancereduction systems 301, 302 were activated at substantially the sametime.

FIG. 4 illustrates an electrode assembly that combines one or more ECGsensing electrodes, a therapy electrode, and redundant impedancereduction systems in a single integrated electrode assembly inaccordance with a further aspect of the present invention. As shown, theelectrode assembly 400 includes a pair of ECG sensing electrodes 412 a,412 b for monitoring the cardiac function of a patient. The electrodeassembly 400 further includes a therapy electrode 414, and at least twoimpedance reduction systems 301, 302, similar to those describedpreviously with respect to FIG. 3. The pair of ECG sensing electrodes412 a, 412 b may be electrically separated from the therapy electrode414, for example, by an insulator. It should be appreciated that inother embodiments, the electrode assembly 400 may include only a singleECG sensing electrode, while in other embodiments, more than two ECGsensing electrodes may be provided. In such alternative embodiments, thenumber and placement of ECG sensing electrodes and may vary from thatshown in FIG. 4. In yet a further embodiment, the integrated electrodeassembly can include additional sensors 416, other than the one or moreECG sensing electrodes and the therapy electrode, that are capable ofmonitoring other physiological parameters of a patient, such as bloodpressure, heart rate, thoracic impedance, pulse oxygen level,respiration rate, heart sounds, etc.

The electrode assembly 400 may be worn on the patient's body such thatone of the pair of ECG sensing electrodes 412 a, 412 b is disposedapproximately in the center of the patient's torso, and the other of thepair of ECG sensing electrodes 412 a, 412 b is disposed on the side ofthe patient's torso. For example, as shown in FIG. 5, the electrodeassembly 400 may be worn on the front of the patient's torso, so thatthe ECG sensing electrode 412 a is disposed approximately in the centerof the patient's chest, and the other ECG sensing electrode 412 b isdisposed on the patient's side. A second electrode assembly 400′ may beworn on the back of the patient's torso to provide a second pair of ECGsensing electrodes 412 a′, 412 b′, so that one of the ECG sensingelectrodes (e.g., ECG sensing electrode 412 a′) of the second pair ofECG sensing electrodes 400′ is disposed approximately in the center ofthe patient's back, and the other ECG sensing electrode (e.g., ECGsensing electrode 412 b′) of the second pair of ECG sensing electrodes400′ is disposed on the patient's side opposite the other ECG sensingelectrode (e.g., ECG sensing electrode 412 b) of the first pair of ECGsensing electrodes 412 a, 412 b, as shown in FIG. 5. Such an arrangementprovides a front-to-back pairing of ECG sensing electrodes (e.g., 412 a,412 a′) and a side-to-side pairing of ECG sensing electrodes (e.g., 412b, 412 b′). It should be appreciated that other placements for the firstelectrode assembly 400 and the second electrode assembly 400′ mayalternatively be used. For example, the first electrode assembly 400 maybe placed on one side of the patient's torso, and the second electrodeassembly 400′ placed on the other side of the patient's torso to provideside-to-side pairings of ECG sensing electrodes.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

What is claimed is:
 1. A medical device comprising: a first impedancereduction system configured to dispense a first amount of a firstelectrically conductive gel proximate a person's skin; and a secondimpedance reduction system configured to dispense a second amount of asecond electrically conductive gel proximate the person's skinresponsive to a determination that the first impedance reduction systemdid not dispense the first amount of the first electrically conductinggel.
 2. The medical device of claim 1, further comprising a therapyelectrode including an electrically conductive layer having a surfaceconfigured to be placed adjacent the person's skin, wherein the firstimpedance reduction system is configured to dispense the first amount ofthe first electrically conductive gel onto the surface of theelectrically conductive layer in response to a first activation signal.3. The medical device of claim 2, wherein the second impedance reductionsystem is configured to dispense the second amount of the secondelectrically conductive gel onto the surface of the electricallyconductive layer in response to a second activation signal that isdistinct from the first activation signal.
 4. The medical device ofclaim 3, further comprising a control unit configured to send the secondactivation signal responsive to a determination that the first impedancereduction system failed to operate.
 5. The medical device of claim 3,wherein the second impedance reduction system is further configured todispense the second amount of the second electrically conductive gel inresponse to the second activation signal and independent of whether thefirst impedance reduction system dispenses the first amount of the firstelectrically conductive gel in response to the first activation signal.6. The medical device of claim 2, wherein the electrically conductivelayer has a plurality of apertures formed therethrough, the plurality ofapertures including a first plurality of apertures and a secondplurality of apertures, wherein the first impedance reduction system isconfigured to dispense the first amount of the first electricallyconductive gel through the first plurality of apertures, and wherein thesecond impedance reduction system is configured to dispense the secondamount of the second electrically conductive gel through the secondplurality of apertures.
 7. The medical device of claim 1, wherein thesecond electrically conductive gel has at least one property differentfrom the first electrically conductive gel.
 8. The medical device ofclaim 1, wherein the second electrically conductive gel has a longerdrying time than the first electrically conductive gel.
 9. The medicaldevice of claim 1, wherein the medical device is a wearable medicaldevice and the first impedance reduction system and the second impedancereduction system are disposed in the wearable medical device.
 10. Amedical device comprising: at least one therapy electrode disposed in awearable medical device, the at least one therapy electrode having anelectrically conductive surface configured to provide electrical therapyto a person, the at least one therapy electrode having at least onefirst aperture and at least one second aperture formed in theelectrically conductive surface of the at least one therapy electrode; afirst impedance reduction system configured to dispense a first amountof a first electrically conductive gel out the at least one firstaperture and onto the electrically conductive surface of the at leastone therapy electrode in response to a first activation signal; a secondimpedance reduction system configured to dispense a second amount of asecond electrically conductive gel out the at least one second apertureand onto the electrically conductive surface of the at least one therapyelectrode in response to a second activation signal that is distinctfrom the first activation signal; and a control unit configured to sendthe second activation signal to the second impedance reduction systemresponsive to a determination that the first impedance reduction systemfailed to dispense the first amount of the first electrically conductivegel.
 11. The medical device of claim 10, wherein the first impedancereduction system and the second impedance reduction system are disposedon the at least one therapy electrode.
 12. The medical device of claim10, wherein the at least one first aperture includes a plurality offirst apertures and the at least one second aperture includes aplurality of second apertures.
 13. The medical device of claim 12,wherein the at least one therapy electrode includes a first plurality ofgel reservoirs containing the first electrically conductive gel and asecond plurality of gel reservoirs containing the second electricallyconductive gel, each respective gel reservoir of the first plurality ofgel reservoirs having an outlet in fluid communication with a respectiveaperture of the plurality of first apertures, and each respective gelreservoir of the second plurality of gel reservoirs having an outlet influid communication with a respective aperture of the plurality ofsecond apertures.
 14. The medical device of claim 13, wherein the firstimpedance reduction system includes a first fluid pressure source influid communication with the first plurality of gel reservoirs andconfigured to receive the first activation signal and force a firstfluid under pressure into the first plurality of gel reservoirs inresponse to receiving the first activation signal, and wherein thesecond impedance reduction system includes a second fluid pressuresource in fluid communication with the second plurality of gelreservoirs and configured to receive the second activation signal andforce a second fluid under pressure into the second plurality of gelreservoirs in response to receiving the second activation signal. 15.The medical device of claim 10, wherein the first impedance reductionsystem includes a at least one first gel reservoir sealed by a membrane,and the second impedance reduction system includes at least one secondgel reservoir sealed by a membrane.
 16. The medical device of claim 15,wherein the first impedance reduction system includes a first fluidpressure source configured to pressurize a first fluid in response tothe first activation signal and the second impedance reduction systemincludes a second fluid pressure source configured to pressurize asecond fluid in response to the second activation signal, the membraneof the at least one first gel reservoir and the membrane of the at leastone second gel reservoir constructed to rupture in response to thepressure of the first fluid and the second fluid, respectively.
 17. Amethod of reducing impedance between a medical device and a person'sskin, comprising acts of: sending a first activation signal to a firstimpedance reduction system configured to dispense, in response toreceipt of the first activation signal, a first amount of a firstelectrically conductive gel onto a surface of an electrode configured tobe disposed adjacent the person's skin; and sending, responsive to adetermination that the first impedance reduction system did not dispensethe first amount of the first electrically conductive gel, a secondactivation signal that is distinct from the first activation signal to asecond impedance reduction system configured to dispense, in response toreceipt of the second activation signal, a second amount of a secondelectrically conductive gel onto the surface of the electrode.
 18. Themethod of claim 17, wherein the act of sending the second activationsignal is performed independent of whether the first impedance reductionsystem dispensed the first amount of the first electrically conductivegel onto the surface of the electrode.
 19. The method of claim 18,wherein the act of sending the first activation signal is performed at afirst time, and wherein the act of sending the second activation signalis performed at a second time that is substantially after the first timeto maintain an impedance between the electrode and the patient's skin ina reduced state for an extended duration.
 20. The method of claim 17,wherein the surface of the electrode includes an electrically conductivelayer having a first plurality of apertures, and the method furthercomprises dispensing the first amount of the first electricallyconductive gel through the first plurality of apertures onto theelectrically conductive layer in response to the first activationsignal.
 21. The method of claim 20, wherein the electrically conductivelayer includes a second plurality of apertures, and the method furthercomprises dispensing the second amount of the second electricallyconductive gel through the second plurality of apertures onto theelectrically conductive layer in response to the second activationsignal.
 22. The method of claim 17, further comprising activating afirst fluid pressure source in fluid communication with at least onefirst gel reservoir to force the first amount of the first electricallyconductive gel onto the surface of the electrode in response toreceiving the first activation signal.
 23. The method of claim 22,further comprising activating a second fluid pressure source in fluidcommunication with at least one second gel reservoir to force the secondamount of the second electrically conductive gel onto the surface of theelectrode in response to receiving the second activation signal.